Radiation emitter device having an encapsulant with different zones of thermal conductivity

ABSTRACT

A radiation emitting device of the present invention includes at least one radiation emitter, first and second electrical leads electrically coupled to the radiation emitter, and an integral encapsulant configured to encapsulate the radiation emitter and a portion of the first and second electrical leads. The encapsulant has at least a first zone and a second zone, where the second zone exhibits at least one different characteristic from the first zone. Such different characteristics may be a physical, structural, and/or compositional characteristic. Preferably, the at least one different characteristic includes at least one of the following: mechanical strength, thermal conductivity, thermal capacity, coefficient of thermal expansion, specific heat, oxygen and moisture impermeability, adhesion, and transmittance with respect to radiation emitted from the radiation emitter. The radiation emitter may be in a form of an emitter, and is preferably an LED.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) on U.S.Provisional Patent Application No. 60/265,489, entitled “RADIATIONEMITTER DEVICES AND METHOD OF MAKING THE SAME,” filed on Jan. 31, 2001,by John K. Roberts et al.

This application is also a continuation-in-part of U.S. patentapplication Ser. No. 09/426,795, filed on Oct. 22, 1999, entitled“SEMICONDUCTOR RADIATION EMITTER PACKAGE,” by John K. Roberts et al.,now U.S. Pat. No. 6,335,548, which claims priority under 35 U.S.C.§119(e) on U.S. Provisional Patent Application No. 60/124,493, entitled“SEMICONDUCTOR RADIATION EMITTER PACKAGE,” filed on Mar. 15, 1999, byJohn K. Roberts et al. The entire disclosure of each of the above-notedapplications is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention generally relates to radiation emitter devicessuch as, for example, light emitting diode (LED) packages, to methods ofmaking radiation emitter devices, and to opto-electronic emitterassemblies incorporating optical radiation emitter devices.

As used herein, the term “discrete opto-electronic emitter assembly”means packaged radiation emitter devices that emit ultraviolet (UV),visible, or infrared (IR) radiation upon application of electricalpower. Such discrete opto-electronic emitter assemblies include one ormore radiation emitters. Radiation emitters, particularly opticalradiation emitters, are used in a wide variety of commercial andindustrial products and systems, and accordingly come in many forms andpackages. As used herein, the term “optical radiation emitter” includesall emitter devices that emit visible light, near IR radiation, and UVradiation. Such optical radiation emitters may be photoluminescent,electroluminescent, or another type of solid state emitter.Photoluminescent sources include phosphorescent and fluorescent sources.Fluorescent sources include phosphors and fluorescent dyes, pigments,crystals, substrates, coatings, and other materials.

Electroluminescent sources include semiconductor optical radiationemitters and other devices that emit optical radiation in response toelectrical excitation. Semiconductor optical radiation emitters includelight emitting diode (LED) chips, light emitting polymers (LEPs),organic light emitting devices (OLEDs), polymer light emitting devices(PLEDs), etc.

Semiconductor optical emitter components, particularly LED devices, havebecome commonplace in a wide variety of consumer and industrialopto-electronic applications. Other types of semiconductor opticalemitter components, including OLEDs, LEPs, and the like, may also bepackaged in discrete components suitable as substitutes for conventionalinorganic LEDs in many of these applications.

Visible LED components of all colors are used alone or in small clustersas status indicators on such products as computer monitors, coffeemakers, stereo receivers, CD players, VCRs, and the like. Suchindicators are also found in a diversity of systems such as instrumentpanels in aircraft, trains, ships, cars, trucks, minivans and sportutility vehicles, etc. Addressable arrays containing hundreds orthousands of visible LED components are found in moving-message displayssuch as those found in many airports and stock market trading centersand also as high brightness large-area outdoor television screens foundin many sports complexes and in some urban billboards.

Amber, red, and red-orange emitting visible LEDs are used in arrays ofup to 100 components in visual signaling systems such as vehicle centerhigh mounted stop lamps (CHMSLs), brake lamps, exterior turn signals andhazard flashers, exterior signaling mirrors, and for roadwayconstruction hazard markers. Amber, red, and blue-green emitting visibleLEDs are increasingly being used in much larger arrays of up to 400components as stop/slow/go lights at intersections in urban and suburbanintersections.

Multi-color combinations of pluralities of visible colored LEDs arebeing used as the source of projected white light for illumination inbinary-complementary and ternary RGB illuminators. Such illuminators areuseful as vehicle or aircraft maplights, for example, or as vehicle oraircraft reading or courtesy lights, cargo lights, license plateilluminators, backup lights, and exterior mirror puddle lights. Otherpertinent uses include portable flashlights and other illuminatorapplications where rugged, compact, lightweight, high efficiency,long-life, low voltage sources of white illumination are needed.Phosphor-enhanced “white” LEDs may also be used in some of theseinstances as illuminators.

IR emitting LEDs are being used for remote control and communication insuch devices as VCR, TV, CD and other audio-visual remote control units.Similarly, high intensity IR-emitting LEDs are being used forcommunication between IRDA devices such as desktop, laptop, and palmtopcomputers; PDAs (personal digital assistants); and computer peripheralssuch as printers, network adapters, pointing devices (“mice,”trackballs, etc.), keyboards and other computers. IR LED emitters and IRreceivers also serve as sensors for proximity or presence in industrialcontrol systems, for location or orientation within such opto-electronicdevices such as pointing devices and optical encoders, and as read headsin such systems as barcode scanners. IR LED emitters may also be used ina night vision system for automobiles.

Blue, violet, and UV emitting LEDs and LED lasers are being usedextensively for data storage and retrieval applications such as readingand writing to high-density optical storage disks.

For discrete LED devices and other discrete (“packaged”) opto-electronicemitters, increased performance is dependent substantially uponincreased reliable package power capacity, reduced package thermalresistance, and reduced susceptibility of the package to damage duringauto-insertion, soldering and other circuit or system manufacturingoperations.

Keeping discrete opto-electronic emitters cool during operation enhancesperformance in several ways. The efficiency of the emitter usuallydecreases in relation to increased operating temperature and increasesin relation to reduced operating temperature. Conversely, emitterefficiency typically increases in relation to reduced internal operatingtemperature. The reliability of the emitter and life of the materialsand sub-components comprising it usually improves in relation todecreased operating temperature. The consistency of the emitter'semission spectra is usually improved in relation to decreased or moreconsistent operating temperature. The decay life of the emitter isusually improved in relation to reduced operating temperature. For theseand other reasons, it is clearly beneficial to employ novel mechanismsfor reducing the operating temperature of discrete opto-electronicemitters.

While the ambient environmental temperature is an external factor thatcannot always be controlled, the temperature rise of the device abovethe ambient temperature is determined mainly by the device's thermalresistance and operating power.

Unfortunately, most discrete opto-electronic emitters exhibit acharacteristic contravening to the goal of reduced internal operatingtemperature. In short, these types of devices usually emit greateramounts of useful radiation in proportion to increased power up to somepractical limit of the package or constituent materials orsubcomponents. Thus, for applications where more radiation is useful(i.e., almost all applications known), it is beneficial to drive thedevice at the highest power consistent with device and systemreliability and consistent with the power-radiation characteristics ofthe device. However, increased power in devices with finite (positive,non-zero) thermal resistance results in elevated internal operatingtemperatures.

It would be advantageous then to reduce internal operating temperaturewithout having to reduce device power, or alternately to maintaininternal operating temperature while increasing device power. This canbe accomplished by reducing the device thermal resistance.

Billions of LED components are used in applications such as those citedabove, in part because relatively few standardized LED configurationsprevail and due to the fact that these configurations are readilyprocessed by the automated processing equipment used almost universallyby the world's electronic assembly industries. Automated processing viamainstream equipment and procedures contributes to low capital cost, lowdefect rates, low labor cost, high throughput, high precision, highrepeatability and flexible manufacturing practices. Without theseattributes, the use of LEDs becomes cost prohibitive or otherwiseunattractive from a quality standpoint for most high-volumeapplications.

Two of the most important steps in modern electronic assembly processesare high-speed automated insertion and mass-automated soldering.Compatibility with automatic insertion or placement machines and one ormore common mass-soldering process are critical to large-scalecommercial viability of discrete semiconductor optical emitters(including LEDs).

Thus, the vast majority of LEDs used take the form of discrete-packagedTHD (Through Hole Device) or SMD (Surface Mount Device) components.These configurations primarily include radial-lead THD configurationsknown as “5 mm,” “T-1,” and “T-1¾” or similar devices with rectangularshapes, all of which are readily adapted onto tape-and-reel,tape-and-ammo, or other standardized packaging for convenient shipment,handling, and high-speed automated insertion into printed circuit boardson radial inserters. Other common discrete THD LED packages includeaxial components such as the “polyLED,” which are readily adapted ontotape and reel for convenient shipment, handling, and high-speedautomated insertion into printed circuit boards on axial inserters.Common SMD LED components such as the “TOPLED®” and Pixar are similarlypopular as they are readily adapted into blister-pack reels forconvenient shipment, handling, and high-speed automated placement ontoprinted circuit boards with chip shooters.

Soldering is a process central to the manufacture of most conventionalcircuit assemblies using standardized discrete electronic devices,whether THD or SMD. By soldering the leads or contacts of a discreteelectronic component such as an LED to a printed circuit board, thecomponent becomes electrically connected to electrically conductivetraces on the PCB and also to other proximal or remote electronicdevices used for supplying power to, controlling or otherwiseinteracting electronically with, the discrete electronic device.Soldering is generally accomplished by wave solder, IR reflow solder,convective IR reflow solder, vapor phase reflow solder, or handsoldering. Each of these approaches differ from one another, but theyall produce substantially the same end effect—inexpensive electricalconnection of discrete electronic devices to a printed circuit board byvirtue of a metallic or inter-metallic bond. Wave and reflow solderprocesses are known for their ability to solder a huge number ofdiscrete devices en masse, achieving very high throughput and low cost,along with superior solder bond quality and consistency.

Widely available cost-effective alternatives to wave solder and reflowsolder processes for mass production do not presently exist. Handsoldering suffers from inconsistency and high cost. Mechanicalconnection schemes are expensive, bulky, and generally ill-suited forlarge numbers of electrical connections in many circuits. Conductiveadhesives such as silver-laden epoxies may be used to establishelectrical connections on some circuit assemblies, but these materialsare more costly and expensive to apply than solder. Spot soldering withlasers and other selective-solder techniques are highly specialized forspecific configurations and applications and may disrupt flexiblemanufacturing procedures preferred in automated electronic circuitassembly operations. Thus, compatibility with wave solder or reflowsolder processes are desirable properties of a semiconductor opticalemitter component. The impact of this property is far reaching, becausethese solder operations can introduce large thermal stresses into anelectronic component sufficient to degrade or destroy the component.Thus an effective semiconductor optical emitter component must beconstructed in such a fashion as to protect the device's encapsulationand encapsulated wire bonds, die attach and chip from transient heatexposure during soldering.

Conventional solder processes require that the ends of the leads of thedevice (below any standoff or at a point where the leads touchdesignated pads on the PCB) be heated to the melting point of the solderfor a sustained period. This profile can include temperature excursionsat the device leads as high as 230-300 degrees C. for as long as 15seconds. Given that the leads of the device are normally constructed ofplated metals or alloys such as copper or steel, this high temperaturetransient poses no problems for the leads themselves. The probleminstead is the ability of these leads to conduct heat along their lengthinto the encapsulated body of the device. Since these heated leads arein contact with the interior of the body of the device, they temporarilyraise the local internal temperature of the device during solderprocessing. This can harm the somewhat delicate encapsulation,encapsulated wire bonds, die attach and chip. This phenomenon representsone of the fundamental limitations of low-cost, opto-electronicsemiconductor devices today.

Keeping the body of an electronic component from rising excessivelyabove the glass transition temperature of its encapsulating materialduring solder processing is critical, since the Coefficient of ThermalExpansion of polymer encapsulating materials rises dramatically abovetheir glass transition points, typically by a factor of two or more.Polymers will increasingly soften, expand and plastically deform abovetheir glass transition points. This deformation from polymer phasetransition and thermal expansion in encapsulants can generate mechanicalstress and cumulative fatigue severe enough to damage a discretesemiconductor device, resulting in poor performance of the device and alatent predisposition toward premature field failure. Such damagetypically consists of: 1) fatigue or fracture of electrical wire bonds(at the chip bond pads or at the lead-frame); 2) partial delamination ordecomposition of die-attach adhesive; 3) micro-fracture of the chipitself; and 4) degradation of the device encapsulant, especially nearthe entry points of the leads into the encapsulant, and a compromisedability to seal out environmental water vapor, oxygen, or other damagingagents.

With regard to such thermal vulnerability, a crucial difference must berecognized between encapsulating materials suitable for non-opticalelectronic devices and those suitable for optical devices. Theencapsulants used for non-optical devices may be opaque, whereas thoseused in constructing opto-electronic emitters and receivers must besubstantially transparent in the operating wavelength band of thedevice. The side effects of this distinction are subtle and far ranging.

Since there is no need for transparency in non-optical devices,encapsulating materials for non-optical semiconductor devices mayinclude a wide range of compositions containing a variety of opaquepolymer binders, cross-linking agents, fillers, stabilizers and thelike. Compositions of this type, such as heavily filled epoxy, maypossess high glass transition temperatures (T_(g)), low thermalexpansion coefficients (C_(te)), and/or elevated thermal conductivitysuch that they are suitable for transient exposures up to 175 degrees C.Opaque ceramic compositions may be thermally stable up to severalhundred degrees C., with no significant phase transition temperatures toworry about, extremely low C_(te) and elevated thermal conductivity. Forthese reasons, exposure of conventional, opaque encapsulation materialsfor non-optical devices to electrical leads heated to 130 degrees C. ormore for 10 seconds or so (by a solder wave at 230-300 degrees C.) isnot normally a problem.

However, the need for optical transparency in encapsulants foropto-electronic emitters and receivers obviates use of mosthigh-performance polymer-filler blends, ceramics and composites that aresuitable for non-optical semiconductors. Without the presence ofinorganic fillers, cross-linking agents or other opaque additives, theclear polymer materials used to encapsulate most opto-electronic devicesare varieties of epoxies having relatively low T_(g) values, greaterC_(te), and low thermal conductivity. As such, they are not suitable forexposure to transient temperature extremes greater than about 130degrees C., such as occurs during normal soldering.

To compensate for the potentially severe effects of damage from solderprocessing, prior art opto-electronic devices have undertaken a varietyof improvements and compromises. The most notable improvement has beenthe relatively recent introduction of clear epoxies for encapsulationcapable of enduring temperatures 10 to 20 degrees C. higher than thosepreviously available (up to 130 degrees C. now versus the previous 110degrees C.). While useful, this has only partially alleviated theproblems noted—the newest materials in use still fall 50 degrees C. ormore short of parity with conventional non-optical semiconductorencapsulation materials.

The most common compromise used to get around the transient temperaturerise problem associated with soldering is to simply increase the thermalresistance of the electrical leads used in the device construction. Byincreasing the thermal resistance of these solderable leads, the heattransient experienced within the device body during soldering isminimized. Such an increase in thermal resistance can typically beaccomplished in the following manner without appreciably affecting theelectrical performance of the leads: 1) using a lead material with lowerthermal conductivity (such as steel); 2) increasing the stand-off lengthof the leads (distance between solder contact and the device body); or3) decreasing the cross-sectional area of the leads.

Using these three techniques, prior art devices have been implementedwith elevated thermal resistance of the electrical leads to provide thedesired protection from the solder process.

While effective at protecting prior art devices from thermal transientsassociated with soldering, there are limits to this approach,particularly in the application of high power semiconductoropto-electronic emitters. Increased lead thermal resistance results inelevated internal operating temperatures in prior art devices, severelycompromising operational performance and reliability of these devices.The soldered electrical leads of most prior art LED devices conductpower to the device and serve as the primary thermal dissipation pathfor heat created within the device during operation. Thus the electricalleads in prior art devices must be configured to possess thermalresistance as low as possible to facilitate heat extraction duringnormal operation. Radiation and natural convection from prior artdevices play only a minor role in transferring internal heat to ambient,and thermal conduction through their encapsulating media is severelyimpeded by the low thermal conductivity of the optical materials used.Therefore, the electrically and thermally conductive metal leads mustextract a majority of the heat to ambient by the mechanism ofconduction. Greater thermal resistance in the solderable pins of thesedevices, necessary to protect the device from the transient thermaleffects of soldering operations, therefore causes a higher internaltemperature rise within the encapsulated device body during operation.

The maximum temperature rise of a portion of the device body in contactwith the semiconductor emitter under steady state is approximately equalto the product of the power dissipation of the emitter and the thermalresistance between the emitter and the ambient environment.

As previously discussed, severe consequences will result if the deviceinternal temperature rises substantially above the encapsulant T_(g)value. Above this temperature, the C_(te) of the encapsulant typicallyincreases very rapidly, producing great thermo-mechanical stress andcumulative fatigue at the LED wire bond and die attach. For most mobileapplications such as automobiles, aircraft and the like, ambienttemperatures commonly reach 80 degrees C. With encapsulation maximumoperating temperatures in the range of 130 degrees C., anopto-electronic emitter for these applications must therefore limit itsoperational ΔT to an absolute maximum of about 50 degrees C. This limitsthe power that can be dissipated in a given component, and in turnlimits the current that may be passed through the component. Since theemitted flux of semiconductor optical emitters are typicallyproportional to the electrical current passed through them, limitationsupon maximum electrical current also create limitations on fluxgenerated.

Thus, it would be advantageous to reduce internal operating temperaturewithout having to reduce device power, or alternately to maintaininternal operating temperature while increasing device power by means ofreducing the device thermal resistance without increasing devicevulnerability to transient thermal processing damage from soldering.

Other prior art devices have avoided these constraints, but haveachieved high performance only by ignoring the needs of standardized,automated electronic assembly operations and adopting configurationsincompatible with these processes. Still other prior art devices haveachieved high performance by employing unusually expensive materials,sub-components, or processes in their own construction.

For example, one prior art approach that has been used to overcome theselimitations uses hermetic semiconductor packaging, hybrid chip-on-boardtechniques, exotic materials such as ceramics, KOVAR and glass, orcomplex assemblies instead of or in addition to polymer encapsulation.While relevant for certain high-cost aerospace and telecommunicationsapplications (where component cost is not a significant concern), suchdevices require expensive materials and unusual assembly processes. Thisresults in high cost and restricted manufacturing capacity—both of whicheffectively preclude the use of such components in mass-marketapplications. The devices disclosed in U.S. Pat. No. 4,267,559 issued toJohnson et al. and U.S. Pat. No. 4,125,777 issued to O'Brien et al.illustrate good examples of this.

The Johnson et al. patent discloses a device which includes both a TO-18header component and a heat coupling means for mounting an LED chipthereto and transferring internally generated heat to external heatdissipating means. The header consists of several components, includinga KOVAR member, insulator sleeves and electrical posts, and ismanufactured in a specialized process to ensure that the posts areelectrically insulated as they pass through the header. The heatcoupling means is a separate component from the header and is composedof copper, copper alloys, aluminum or other high thermal conductivitymaterials. According to the teachings of Johnson et al., the KOVARheader subassembly and copper heat coupling means must be bondedtogether with solder or electrically conductive adhesive for electricalcontinuity, allowing flow of electrical current into the heat couplingmeans and subsequently into the LED chip. Furthermore, the header andheat coupling means of the Johnson et al. patent are made of completelydissimilar materials and must be so because of their unique roles in thedescribed assembly. The header must be made of KOVAR in order that itmay have a similar coefficient of thermal expansion to the insulatorsleeves that run through it. At least one such sleeve is necessary toelectrically isolate electrical pins from the header itself. However,KOVAR has relatively low thermal conductivity, necessitating theinclusion of a separate heat coupling means made of a material such ascopper with a higher thermal conductivity. Since the header is a complexsubassembly itself and is made of different materials than the heatcoupling means, it must be made separately from the heat coupling meansand then later attached to the heat coupling means with solder or anelectrically conductive adhesive.

LED devices made similarly to the teachings of the Johnson et al. patentare currently being marketed in specialized forms similar to a TO-66package. These devices are complex and typically involve insulated pinand header construction and/or include specialty sub-components such asceramic isolation sheets within them.

Another approach which has been used to avoid damage to opto-electronicemitters from soldering has been to prohibit soldering of the componentaltogether or to otherwise require use of laser spot soldering or otherunusual electrical attachment method. This can allow construction of adevice with low thermal resistance from the semiconductor emitter withinto the electrical pins without danger of device damage from solderingoperations. The SnapLED 70 and SnapLED 150 devices made by HewlettPackard illustrate this approach. In these devices, electricalconnections are made to circuitry by mechanically stamping the leads toa simple metal circuit rather than soldering. The resultant devices arecapable of continuous power dissipation as high as 475 mW at roomtemperature. This configuration, however, may complicate integration ofsuch components with electronic circuits having higher complexity—suchcircuits are conventionally made using printed circuit boards, automatedinsertion equipment, and wave or reflow solder operations.

A final approach is illustrated by an LED package called the SuperFluxpackage (also known as the “Piranha”), available from Hewlett Packard.The SuperFlux device combines moderate thermal resistance between theencapsulated chip and the solder standoff on the pins with a high-gradeoptical encapsulant and specialized chip materials and optical design.It achieves a moderate power dissipation capability without resorting toa non-solderable configuration such as the SnapLED. However, there areseveral significant problems with this configuration that inhibit itsbroader use.

The package geometry of the SuperFlux package renders it incompatiblewith conventional high-speed THD radial or axial insertion machinery orby SMT chip shooters known to the present inventors. Instead, it must beeither hand-placed or placed by expensive, slow, robotic odd-forminsertion equipment. The SuperFlux package geometry is configured foruse as an “end-on” source only—no readily apparent convenient lead-bendtechnique can convert this device into a 90-degree “side-looker” source.The moderate thermal resistance of the solderable pins of this deviceand relatively low heat capacity may leave it vulnerable to damage frompoorly controlled solder processes. It may be inconvenient or costly forsome electronic circuit manufacturers to control their solderingoperations to the degree needed for this configuration. Finally, thereis no convenient mechanism known to the inventors to outfit a SuperFluxpackage with a conventional active or passive heat sink.

A principle factor impeding further application of these and other LEDdevices in signaling, illumination and display applications is thatthere is not currently available a device that has a high powercapability with high emitted flux where the device is easily adaptableto automated insertion and/or mass-soldering processes. Theselimitations have either impeded the practical use of LEDs in manyapplications requiring high flux emission, or they have mandated the useof arrays of many LED components to achieve desired flux emission.

Conventional “5 mm” or “T 1¾” devices have a high thermal resistance,typically in excess of 240 degrees C. per watt and usually are limitedby clear encapsulation materials that lead to unreliability if theemitter in the device is operated continuously, routinely or cyclicallyabove 130 degrees C. (less for any but the best materials clearavailable). With typical ambient temperatures commonly exceeding 85degrees C. in the automotive environment, the temperature rise in thesedevices must be limited to 45 degrees C. in order to properly avoidthese material limits. This means that the device power must be limitedto approximately 0.18 W. With a reasonable design tolerance of 33percent to accommodate manufacturing variances, the practical reliablepower limit of this device must be approximately 0.12 W. This is not alot of power, and the emitted flux of these devices is thus limited. Toovercome this, many of these devices are often used in combination toproduce the luminous or radiant flux needed for an application (e.g., upto 50 for an automotive CHSML, up to 400 for a traffic signal lamp).

Hewlett Packard's SuperFlux or Piranha devices have a lower thermalresistance than “5 mm” or “T 1¾” devices, typically around 145 degreesC. per watt. As with “5 mm” or “T 1¾” devices, SuperFlux or Piranhadevices usually are limited by clear encapsulation materials that leadto unreliability if the emitter in the device is operated continuously,routinely, or cyclically above 130 degrees C. (less for any but the bestmaterials clear available). With typical ambient temperatures commonlyexceeding 85 degrees C. in the automotive environment, the temperaturerise in these devices must be limited to 45 degrees C. in order toproperly avoid these material limits. This means that the device powermust be limited to approximately 0.3 W. Because these devices areattached subsequently with thermally stressful wave or other solderoperations, and because their thermal resistance from lead to junctionis reduced, they are more susceptible to damage during processing intocircuits. Thus, a higher design tolerance of 40 percent should be usedto accommodate manufacturing variances and increased susceptibility, andthe practical reliable power limit of this device must be approximately0.18 W. This is a substantial increase (33 percent) compared to “5 mm”or “T 1¾” devices, it still is not a lot of power and the emitted fluxof these devices is thus also limited. To overcome this, many of thesedevices are often used in combination to produce the luminous or radiantflux needed for an application (e.g., up to 30 for an automotive CHSML).

Hewlett Packard's SnapLED devices have a lower thermal resistance than“5 mm” or “T 1¾” or SuperFlux or Piranha devices, as low as 100 degreesC. per watt. As with “5 mm” or “T 1¾” or SuperFlux, Piranha, or SnapLEDdevices usually are limited by clear encapsulation materials that leadto unreliability if the emitter in the device is operated continuously,routinely, or cyclically above 130 degrees C. (less for any but the bestmaterials clear available). With typical ambient temperatures commonlyexceeding 85 degrees C. in the automotive environment, the temperaturerise in these devices must be limited to 45 degrees C. in order toproperly avoid these material limits. This means that the device powermust be limited to approximately 0.45 W. As noted above, because thethermal resistance of these devices from lead to junction is so low,they cannot be soldered by conventional means without being damaged.This severely limits their utility, but they still are suitable for someapplications. Because these devices are attached subsequently withmechanically stressful clinching operations, they remain susceptible todamage during processing operations. Thus, a higher design tolerance of40 percent should be used to accommodate manufacturing variances andpotentially increased processing damage susceptibility, and thepractical reliable power limit of this device must be approximately 0.27W. This is a significant increase compared to “5 mm” or “T 1¾” orSuperFlux or Piranha devices, but it still is not a lot of power (and isachieved at a sacrifice in conventional solderability). To overcome theresulting limited flux from these devices, many are often used incombination to produce the luminous or radiant flux needed for anapplication (e.g., up to 12 for an automotive CHSML and up to 70 for anautomotive rear combination stop/turn/tail lamp).

Surface mount devices such as the TOPLED®, PLCC and Hewlett Packard's“High-Flux” or “Barracuda” devices use dissimilar polymer materials intheir construction, the first in order of assembly being a plasticmaterial that forms the basic structure of the device body and holds thedevice leads together. However, this approach requires that the leadframes be processed initially via insert molding (to emplace the firstsupporting material around the lead frame), then die mounting, then wirebonding and then a second stage of molding. The second stage of moldingmust be the optical molding (to first provide an opportunity for diebonding and wire bonding). Such a design and process are difficult andexpensive to execute with high yield and high quality. Accumulatedvariances would be excessive from the multistage molding scheme,interrupted by die and wire bonding.

An additional problem faced by designers of conventional LED devices isthat the wire bond used to join one of the LED leads to the LED chip canbreak or lose contact with the lead or the chip. Such failure can occur,for example, due to shear forces that are transferred to the wire bondthrough the encapsulant or thermal expansion/contraction of theencapsulant around the wire bond.

The other forms of radiation emitters mentioned above also experienceperformance degradation, damage, increased failure probability, oraccelerated decay if exposed to excessive operating temperatures.

Consequently, it is desirable to provide a radiation emitter device thathas the capacity for higher emission output than conventional LEDdevices while being less susceptible to failure due to a break in thewire bond contact or other defect that may be caused by excessiveoperating temperatures.

Additionally, it is desirable to provide a radiation emitter devicehaving improved emission output over that of conventional LED deviceswhile retaining the same size and shape of the conventional LED devicesso as to facilitate the immediate use of the inventive LED devices inplace of the conventional LED devices while also requiring minimalmodification to the apparatuses that are used to manufacture the LEDdevices.

SUMMARY OF THE INVENTION

Accordingly, it is an aspect of the present invention to provide aradiation emitter device that overcomes the above-noted problems andthat provides improved performance and less vulnerability to fataldamage. According to one embodiment of the present invention, aradiation emitting device comprises at least one radiation emitter,first and second electrical leads electrically coupled to the radiationemitter, and an integral encapsulant configured to encapsulate theradiation emitter and a portion of the first and second electricalleads. The encapsulant has at least a first zone and a second zone. Thesecond zone exhibits at least one different characteristic from thefirst zone. The different characteristic may be a physical, structural,and/or compositional characteristic. For example, the at least onedifferent characteristic may include at least one or more of thefollowing: mechanical strength, thermal conductivity, thermal capacity,specific heat, coefficient of thermal expansion, adhesion, oxygenimpermeability, moisture impermeability, and transmittance for radiationemitted from the radiation emitter.

A method of making a radiation emitting device according to the presentinvention comprises the steps of (1) attaching and electrically couplingat least one radiation emitter to a lead frame to form a subassembly;(2) inserting the subassembly into a mold cavity; (3) partially fillingthe mold cavity with a first encapsulant material; (4) filling theremainder of the mold cavity with a second encapsulant material; and (5)removing the encapsulated subassembly from the mold cavity.

For a wide range of otherwise-conventional discrete opto-electronicemitters, the present invention accomplishes a significant increase inthe reliable package power capacity and accomplishes significantreductions in the package thermal resistance and package damagesusceptibility in novel ways.

These and other features, advantages, and objects of the presentinvention will be further understood and appreciated by those skilled inthe art by reference to the following specification, claims, andappended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a perspective view of a radiation emitter device constructedin accordance with a first embodiment of the present invention;

FIG. 2 is a top view of the radiation emitter device shown in FIG. 1;

FIG. 3 is a cross-sectional view of the radiation emitter device shownin FIGS. 1 and 2 taken along line 3-3′ as represented in FIG. 2;

FIG. 4 is a side view of a lead frame subassembly from which radiationemitter device shown in FIGS. 1-3 may be constructed;

FIG. 5 is a perspective view of the lead frame subassembly shown in FIG.4 inverted and inserted into a mold in accordance with the inventivemethod for making radiation emitter devices;

FIG. 6 is a partial cross-sectional view taken along line 6-6′ in FIG.5, which shows a portion of the lead frame subassembly inverted andinserted into the mold shown in FIG. 5 prior to a first molding step;

FIG. 7 is a partial cross-sectional view taken along line 6-6′ in FIG.5, which shows a portion of the lead frame subassembly inverted andinserted into the mold shown in FIG. 5 after a first molding step;

FIG. 8 is a partial cross-sectional view taken along line 6-6′ in FIG.5, which shows a portion of the lead frame subassembly inverted andinserted into the mold shown in FIG. 5 after a final molding step;

FIG. 9 is a side view of the final lead frame assembly following removalfrom the mold;

FIG. 10 is a flow chart showing the steps of the inventive method forproducing radiation emitter devices in accordance with the presentinvention;

FIG. 11 is a cross-sectional view of a radiation emitter deviceconstructed in accordance with a second embodiment of the presentinvention;

FIG. 12 is a perspective view of a radiation emitter device constructedin accordance with a third embodiment of the present invention;

FIG. 13 is a perspective view of a radiation emitter device constructedin accordance with a fourth embodiment of the present invention;

FIG. 14 is an exploded perspective view of the radiation emitter deviceshown in FIG. 13;

FIG. 15 is a perspective view of a radiation emitter device constructedin accordance with a fifth embodiment of the present invention; and

FIG. 16 is a graph showing the average normalized luminous flux as afunction of applied power for a conventional “T-1¾” LED device and forthe inventive “T-1¾” LED device shown in FIGS. 1-3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numeralswill be used throughout the drawings to refer to the same or like parts.

For purposes of description herein, the terms “upper,” “lower,” “right,”“left,” “rear,” “front,” “vertical,” “horizontal,” “top,” “bottom,” andderivatives thereof shall relate to the invention as viewed by a personlooking directly at the radiation emitting device along the principaloptical axis of the source. However, it is to be understood that theinvention may assume various alternative orientations, except whereexpressly specified to the contrary. It is also to be understood thatthe specific device illustrated in the attached drawings and describedin the following specification is simply an exemplary embodiment of theinventive concepts defined in the appended claims. Hence, specificdimensions, proportions, and other physical characteristics relating tothe embodiment disclosed herein are not to be considered as limiting,unless the claims expressly state otherwise.

Several embodiments of the present invention generally relate to animproved optical radiation emitting device useful in both high and lowpower applications. Such embodiments of the present invention areparticularly well suited for use in limited power applications such asvehicles, portable lamps, and specialty lighting. By vehicles, we meanover-land vehicles, watercraft, aircraft and manned spacecraft,including but not limited to automobiles, trucks, vans, buses,recreational vehicles (RVs), bicycles, motorcycles and mopeds, motorizedcarts, electric cars, electric carts, electric bicycles, ships, boats,hovercraft, submarines, airplanes, helicopters, space stations,shuttlecraft, and the like. By portable lamps, we mean camping lanterns,head or helmet-mounted lamps such as for mining, mountaineering, andspelunking, hand-held flashlights, and the like. By specialty lightingwe mean emergency lighting activated during power failures, fires orsmoke accumulations in buildings, microscope stage illuminators,billboard front-lighting, backlighting for signs, etc. The lightemitting assembly of the present invention may be used as either anilluminator or an indicator. Examples of some of the applications inwhich the present invention may be utilized are disclosed in commonlyassigned U.S. patent application Ser. No. 09/425,792 entitled“INDICATORS AND ILLUMINATORS USING A SEMICONDUCTOR RADIATION EMITTERPACKAGE,” filed on Oct. 29, 2000, by J. Roberts et al., the entiredisclosure of which is incorporated herein by reference.

Some of the embodiments of the present invention provide a highlyreliable, low-voltage, long-lived, light source for vehicles, portablelighting, and specialty lighting capable of producing white light withsufficient luminous intensity to illuminate subjects of interest wellenough to be seen and to have sufficient apparent color and contrast soas to be readily identifiable. Several of the radiation emitter devicesof the present invention may be well suited for use with AC or DC powersources, pulse-width modulated DC power sources, and electronic controlsystems. The radiation emitting devices of the present invention mayfurther be used to emit light of various colors and/or to emitnon-visible radiation such as IR and UV radiation.

As used herein, the term “radiation emitter” and “radiation emittingdevice” shall include any structure that generates and emits optical ornon-optical radiation, while the term “optical radiation emitter” or“optical radiation emitting device” includes those radiation emittersthat emit optical radiation, which includes visible light, near infrared(IR) radiation, and/or ultraviolet (UV) radiation. As noted above,optical radiation emitters may include electroluminescent sources orother solid-state sources and/or photoluminescent or other sources. Oneform of electroluminescent source includes semiconductor opticalradiation emitters. For purposes of the present invention,“semiconductor optical radiation emitters” comprise any semiconductorcomponent or material that emits electromagnetic radiation having awavelength between 100 nm and 2000 nm by the physical mechanism ofelectroluminescence, upon passage of electrical current through thecomponent or material. The principle function of a semiconductor opticalradiation emitter within the present invention is the conversion ofconducted electrical power to radiated optical power. A semiconductoroptical radiation emitter may include a typical IR, visible or UV LEDchip or die well known in the art and used in a wide variety of priorart devices, or it may include any alternate form of semiconductoroptical radiation emitter as described below.

Alternate forms of semiconductor optical radiation emitters which may beused in the present invention are light emitting polymers (LEPs),polymer light emitting diodes (PLEDs), organic light emitting diodes(OLEDs), and the like. Such materials and opto-electronic structuresmade from them are electrically similar to traditional inorganic LEDs,but rely on organic compositions such as derivatives of the conductivepolymer polyaniline for electroluminescence. Such semiconductor opticalradiation emitters are relatively new, but may be obtained from sourcessuch as Cambridge Display Technology, Ltd. of Cambridge, and from Uniaxof Santa Barbara, Calif.

For brevity, the term semiconductor optical radiation emitter may besubstituted with the term LED or the alternate forms of emittersdescribed above or known in the art. Examples of emitters suitable forthe present invention include varieties of LED chips with associatedconductive vias and pads for electrical attachment and that are emissiveprincipally at P-N or N-P junctions within doped inorganic compounds ofAlGaAs, AlInGaP, GaAs, GaP, InGaN, AlInGaN, GaN, SiC, ZnSe and the like.

LEDs are a preferred electroluminescent light source for use in theradiation emitting devices of the present invention because LEDs do notsuffer appreciable reliability or field-service life degradation whenmechanically or electronically switched on and off for millions ofcycles. The luminous intensity and illuminance from LEDs closelyapproximates a linear response function with respect to appliedelectrical current over a broad range of conditions, making control oftheir intensity a relatively simple matter. Finally, recent generationsof AlInGaP, AlGaAs, InGaN, AlInGaN, and GaN LEDs draw less electricalpower per lumen or candela of visible light produced than incandescentlamps, resulting in more cost-effective, compact, and lightweightilluminator wiring harnesses, fuses, connectors, batteries, generators,alternators, switches, electronic controls, and optics. A number ofexamples have previously been mentioned and are incorporated within thescope of the present invention, although it should be recognized thatthe present invention has other obvious applications beyond the specificones mentioned which do not deviate appreciably from the teachingsherein and therefore are included in the scope of this invention.

Another preferred radiation source that may be used in the inventivelight emitting assembly is a photoluminescent source. Photoluminescentsources produce visible light by partially absorbing visible orinvisible radiation and re-emitting visible radiation. Photoluminescentsources are phosphorescent and fluorescent materials, which includefluorescent dyes, pigments, crystals, substrates, coatings, as well asphosphors. Such a fluorescent or phosphorescent material may be excitedby an LED or other radiation emitter and may be disposed within or on anLED device, or within or on a separate optical element, such as a lensor diffuser that is not integral with an LED device. Exemplarystructures using a fluorescent or phosphorescent source are describedfurther below.

FIGS. 1-3 show a radiation emitter device 10 constructed in accordancewith a first embodiment of the present invention. As shown, radiationemitter device 10 includes an encapsulant 12, first and secondelectrical leads 14 and 16, and a radiation emitter 35. Encapsulant 12encapsulates emitter 35 as well as a portion of each of electrical leads14 and 16. Electrical leads 14 and 16 may have optional respectivestandoffs 18 and 20, which are provided to aid in the auto insertion ofthe device when constructed for THD applications.

As best shown in FIG. 3, an upper end of first electrical lead 14extends horizontally outward and defines a reflective cup 36 on whichradiation emitter 35 is preferably attached. Electrical connection of afirst contact terminal of radiation emitter 35 to first electrical lead14 may be made through a die-attach (not shown) or may be made by way ofa wire bond or other connector. Device 10 further includes a wire bond38 or other means for electrically coupling a second contact terminal ofradiation emitter 35 to second electrical lead 16. The upper ends offirst lead 14 and second lead 16 are electrically insulated from anotherby their spaced relation and by the fact that the encapsulant 12 ispreferably made of a material having a relatively high electricalresistance.

The radiation emitter device 10 shown in FIGS. 1-3 is intended to havethe same relative size and shape as a conventional 5 mm/T-1¾ LED deviceor 3 mm T-1device, and accordingly, encapsulant 12 includes a lower rim22 and a flat side 24, which facilitate registration of the radiationemitter device by an auto inserter.

As best shown in FIG. 3, an optional glob-top 40 or other optical orphysical moderator is deposited over radiation emitter 35 and at least aportion of wire bond 38. Glob-top 40 may be made of silicone or silasticand may include an optional phosphor or other photoluminescent material.Use of a glob-top 40 is beneficial to help protect radiation emitter 35and its connection to wire bond 38 during the molding processes. Otheradvantages of providing a glob-top 40 are discussed further below.

The encapsulant 12 of the inventive radiation emitter device 10 includesat least two functional zones 30 and 32 with a transition region 31between zones 30 and 32. Two separate functional zones 30 and 32 areprovided based upon the inventors' recognition that different portionsof an encapsulant may serve different functions from other portions ofthe encapsulant such that the first zone 30 may have at least onedifferent characteristic than the second zone 32 so as to optimizeperformance of the function(s) to be performed by that particular zone.For example, first zone 30 should be at least partially transmissive tothe wavelengths of radiation emitted from radiation emitter 35, whilesecond zone 32 need not be transparent to such wavelengths. This allowsthe radiation emitter device of the present invention to make use of theextraordinary benefits of high performance power semiconductorencapsulation and transfer-molding compounds in the second zone. Thesecharacteristics can include relatively low coefficient of thermalexpansion; relatively high thermal conductivity; relatively high T_(g);relatively high specific heat; relatively low permeability to oxygen,gas, or water vapor; and relatively high physical strength properties.The compounds used for packaging or potting many high-power non-opticalelectronic devices are superior by a large margin in many of thesecategories to those traditionally used for conventional opto-electronicemitters. One of the main reasons for the disparity is that the highperformance materials under discussion are usually opaque mixtures—nottransparent to the band of radiation emitted in discrete opto-electronicemitter devices. The opacity of these functionally attractive materialsis intrinsically linked to their beneficial properties (by virtue of theperformance-enhancing mineral, metal, and metal-oxide fillers, forexample), and thus, these materials had not been previously consideredfor use in opto-electronic components due to their opacity. However, bylimiting the use of such materials to a zone of encapsulant 12 that doesnot require transparency, the present invention enjoys all the benefitsof these material characteristics.

First zone 30 of encapsulant 12 is preferably a substantiallytransparent material to preserve optical performance. First zone 30 mayoptionally be partially diffuse. First zone 30 may be made of anyconventional transparent encapsulant commonly used for opto-electronicemitter devices. First zone 30 of lens 12 preferably covers, envelopes,protects, and supports radiation emitter 35, the die-attach (if present)and a portion of any wire bonds 38 connected to radiation emitter 35.

First zone 30 of encapsulant 12 may be comprised of two or moreportions, with the innermost being a silicone or silastic glob-top 40preapplied to radiation emitter 35 prior to the first stage of moldingof the encapsulant of the present invention. This innermost portion offirst zone 30 may alternately be a high performance epoxy, silicone,urethane, or other polymer material possibly including opticallytranslucent or transparent fillers or diffusants.

First zone 30 of encapsulant 12 is preferably made of a compositioncomprising an optical epoxy mixture that is substantially transparent tothe radiation emitted by radiation emitter 35. However, other clearmaterials may also be used, and the materials need not be transparent inbands outside the primary emission band of the radiation emitter.

Second zone 32 of encapsulant 12 is preferably made of a material thatoptimizes the function of that region of encapsulant 12. As noted above,second zone 12 need not be transparent. However, a specialized functionof zone 32 is generally to minimize catastrophic failure, stress, andaccumulated fatigue from mechanical stresses propagated up electricallyconductive leads 14 and 16. Not only may a material that is bettersuited for this purpose be selected given that it need not betransparent, but also the material may have higher strength properties,including higher tensile and compressional strength, adhesion, and/orcohesion.

Another function served by second zone 32 of encapsulant 12 is to serveas a barrier to oxygen, molecular water vapor, or other reagents thatmay otherwise propagate upward into the device through second zone 32 orthrough the interface between leads 14 and 16 and encapsulant 12. Thus,second zone 32 should effectively protect radiation emitter 35, thedie-attach (if present) wire bond 38, the encapsulated portions of thelead frame plating, and other internal device constituents including anyphotoluminescent material that may be present, from oxygen, molecularwater vapor, and other reagents. Because second zone 32 of encapsulant12 need not be transparent, second zone 32 may be constructed withimproved barrier properties compared to those present in conventionaltransparent encapsulants.

Perhaps one of the more significant different characteristics thatsecond zone 32 may have from first zone 30 is that it may have improvedthermal characteristics. To achieve lower device thermal resistance,second zone 32 preferably has a high thermal conductivity, at least inthe critical region of the device surrounding electrical leads 14 and 16and in thermal coupling to the portion of the leads that supportsradiation emitter 35 (i.e., reflective cup 36, if present). To preserverelatively high thermal resistance protection from soldering operations,the bottom portion of second zone 32 of encapsulant 12 extends no closerto the solderable portion or ends of electrically conductive leads 14and 16 than the standoffs 18 and 20 (if present) or an equivalent pointon the leads destined to remain substantially out of contact with moltensolder during processing, if standoffs are not present.

By forming second zone 32 of encapsulant 12 to have a high heatcapacity, second zone 32 will help suppress transient temperature spikesduring processing or operation. Also, by configuring second zone 32 tohave a low coefficient of thermal expansion, catastrophic failure,stress, and accumulated fatigue from thermal expansion and contractionwithin the device is minimized.

To achieve different functional characteristics for the first and secondzones 30 and 32 of encapsulant 12, the two zones may have differentphysical properties. Such physical properties may be structural orcompositional. Such different structural characteristics may be obtainedusing the same general composition for both first and second zones 30and 32 but by causing a change in grain size or micro-structuralorientation within the two zones. Such structural characteristics may bemodified during the molding process by treating the zones differently byannealing, radiation curing, or other radiation treatment. Further, themicro-structural orientation may be changed by applying a magnetic fieldto one or more of the zones forming encapsulant 12.

In the event two different compositions are utilized to form first andsecond zones 30 and 32, it is preferable that the material compositionsare compatible for molding in the same mold, as is discussed furtherbelow with reference to the inventive process for making a preferredembodiment of the present invention. By integrally molding first andsecond zones 30 and 32, a cohesive bond may be formed at transition 31between zones 30 and 32. Such a cohesive bond is desirable to improvethe strength of the encapsulant as a whole and to prevent oxygen, watervapor, or other reagents from reaching radiation emitter 35 via anyinterface between zones 30 and 32 that otherwise may be present.Further, such a cohesive bond provides continuity of the outer surface.As discussed further below, it is desirable that the compositions usedfor first and second zones 30 and 32 partially intermix at transition31. Transition 31 may be a fairly narrow cross section of encapsulant 12or may be broader and larger if a composition gradient is formed usingthe compositions of first and second zones 30 and 32.

An additional advantage of making second zone 32 of encapsulant 12opaque is that any back-scattering from device 10 may be substantiallyreduced. Such back-scattering may be a problem when a light sensor ismounted in the same housing as radiation emitting device 10, as is oftenthe case when such radiation emitter devices are mounted in anelectrochromic rearview mirror assembly for an automobile.

Having described the general physical structure of the radiationemitting device of the present invention, an inventive method for makingsuch a radiation emitter device is described below. It will beappreciated, however, that radiation emitter device 10 may be made usingother methods.

FIG. 10 shows a flowchart showing the steps and optional steps for theinventive method. Reference will be made to FIG. 10 simultaneously withreferences to FIGS. 4-9, which show various stages of the deviceassembly. The first step 100 in the inventive method is to prepare alead frame. An example of a lead frame is shown in FIG. 4 and isdesignated with reference numeral 52. The lead frame may be made in anyconventional configuration using any conventional techniques. Lead frame52 is preferably made of metal and may be stamped and optionallypost-plated. Lead frame 52 may also undergo optional ultrasonic or othercleaning. As shown in FIG. 4, lead frame 52 includes the first andsecond electrically conductive leads 14 and 16 for a plurality ofradiation emitter devices. Leads 14 and 16 are held together by a firsttie bar 54 and by a second tie bar 56 that extend substantiallyperpendicularly to leads 14 and 16. Lead frame 52 may further includevertical frame members 58 that extend between first and second tie bars54 and 56 at both ends of lead frame 52 and between pairs of leads 14and 16.

Lead frame 52 is also preferably shaped to include a support (preferablya reflective cup 36) at one end of first electrical lead 14. Reflectivecup 36 may be polished or plated to increase its reflectivity.

The next step in the process (step 102) is to attach one or moreradiation emitter 35 to each reflective cup 36 on lead frame 52. In themost preferred embodiment, radiation emitters 35 are LED chips and aredie-bonded with conductive epoxy attach or eutectic bond into/onto eachcup 36 or other support structure in the lead frames. The LED chips, ifused, may be any conventional LED chip or any LED chip or otherradiation emitter subsequently developed. As part of this step, theattach epoxy may optionally be degassed by vacuum and then cured/cooled.This structure may then optionally be subjected to ultrasonic or othercleaning following the above steps.

For the most preferred embodiment, radiation emitters 35 are thenwire-bonded with bond wire to establish the desired conductive path forradiation emitter 35 (step 104). Then, in step 106, an optionalphosphor, glob-top, or other optical or physical moderator is thendeposited on radiation emitter 35. Note that more than one such opticalor physical moderator may be used (e.g., a phosphor can first be appliedand cured/dried followed by a silicone glob-top application). Whateveris applied at this stage is normally then dried and cured (step 108).Optionally, any such optional optical or physical moderator may bedegassed by vacuum prior to proceeding to the next step.

The next step (step 110) involves the optional application of a clearepoxy within the reflective cup 36 followed by an optional degassingstep, which may be performed by vacuum. This optional application ofclear epoxy may be performed to prevent bubbles from forming in andaround the reflective cup during the subsequent molding operation. Theclear epoxy applied may be the same material that is applied during thefirst molding stage described further below. Following step 110, theconstruction of lead frame subassembly 50 (FIG. 4) is complete and suchsubassembly is ready for molding.

The next step (step 112) is thus to invert lead frame subassembly 50 andinsert and register portions of the lead frame subassembly intoencapsulation mold cavities 62 formed in a mold 60. As shown in FIG. 5,the mold preferably includes a plurality of lead frame supports 64 forreceiving and registering lead frame subassembly 50 in a proper positionrelative to mold cavities 62. FIG. 6 shows a cross-sectional view of onesuch mold cavity 62 with a corresponding portion of lead framesubassembly 50 inverted and inserted into cavity 62.

The next step (114) is to perform the first stage of encapsulationwhereby a clear epoxy lens material is dispensed (preferably byinjection) into encapsulant mold cavity 62. Precise metering or feedbackis preferably used to fill the clear epoxy just up to or over theinverted lip of the reflective cup 36 or surfaces of the radiationemitter 35, if for some reason there is no reflective cup in the device.See, for example, FIG. 7. Next, an optional degas step (step 116) isperformed to remove bubbles by vacuum from the clear epoxy. A step (118)of precuring the clear epoxy may then optionally be performed. Thisoptional precure may be just enough of a cure to minimize free mixing ofthe two primary encapsulation materials, but also be not so much as toprevent some mixing. Some minor mixing in the transition boundary 31 isbelieved to be good for homogenous strength, cohesive bonding, etc.

The next step (step 120) is to perform the second stage of encapsulationmolding in which a base epoxy is dispensed within mold cavity 62(preferably by injection) so as to fill the remainder of mold cavity 62.Precise metering or feedback is preferably used to fill just up to thedesigned bottom of the device body or the top of standoffs 18 and 20, ifpresent. FIG. 8 shows an appropriate filled mold cavity 62 following thesecond stage.

After step 120, step 122 is optionally performed whereby the baseencapsulation material is degassed by vacuum to remove any bubbles.

Then, in step 124, the base encapsulation material is cured along withany residual curing of any other previously in-place materials that arenot yet fully cured. Next, in step 126, the nearly finished lead framestructure is ejected from mold 60. An optional post-cure step 128 maythen be performed followed by an optional cleaning/deflash step 130. Theresultant structure is shown in FIG. 9.

The next step is a singulation step 132 whereby second tie bar 56 andvertical lead frame members 58 are cut away from the finished lead frameassembly and first tie bar 54 is cut between first and second electricalleads 14 and 16 for each device as well as between each device. Ifstandoffs are not desired, first tie bar 54 may be removed in itsentirety, otherwise the portions of tie bar 54 that remain may serve asstandoffs 18 and 20.

After the singulation step 132, an optional testing step 134 may beperformed and the device may then be packed and shipped. Variations ofthis method are discussed below with reference to the alternativeembodiments of the invention.

FIG. 11 shows a radiation emitter device 150 constructed in accordancewith a second embodiment of the present invention. Device 150 differsfrom device 10 discussed above in that it includes a plurality ofradiation emitters 35 a and 35 b. Both emitters 35 a and 35 b may bemounted in a common reflective cup 36 or may be mounted in separate cupsprovided on the same or separate leads. Depending upon the electricalconnection and control desired, an additional lead 16 b may be provided.Radiation emitters 35 a and 35 b may be connected in series or inparallel and may be identical or have different constructions so as toemit light of different wavelengths. In one preferred embodiment,emitters 35 a and 35 b emit light of a binary complementary nature toproduce effective white light. LED chips and devices suitable for suchapplication are disclosed in commonly assigned U.S. Pat. No. 5,803,579entitled “ILLUMINATOR ASSEMBLY INCORPORATING LIGHT EMITTING DIODES,”filed on Jun. 16, 1996, by Robert R. Turnbull et al. The entiredisclosure of this patent is incorporated herein by reference.

The base epoxy used to form second zone of encapsulant 12 may bedistinct from the clear lens epoxy used to form first zone 30 not onlyin composition, but additionally or alternatively distinct in one ormore physical properties (spectral transmittance at a wavelength ofinterest, diffuse scattering properties at one or more wavelengths ofinterest, microcrystalline structure, strength, thermal conductivity,C_(TE), T_(g), etc.). The transition zone 31 between first zone 30 andsecond zone 32 may occur at a transition boundary zone 31, which may benarrow (effecting a more abrupt transition in properties) or broad(effecting a more gradual transition or gradient in properties). Asdiscussed above, the distinction between lens epoxy and base epoxy maybe compositional and achieved by using two different material mixturesin the manufacturing process. A narrow transition boundary zone 31between zones 30 and 32 might then be achieved by ensuring twoformulations that are substantially immiscible or by slightly orcompletely precuring one material before the other is added. A broadboundary zone 31 might be achieved by not precuring the first materialcompletely prior to adding the second material and by ensuring theformulae of the two materials allow some mixing at their boundary.

In the event that a distinction desired between lens epoxy and baseepoxy is not primarily a compositional distinction, but rather aphysical distinction, then alternate means may be used to accomplishthis, if the above-noted means is insufficient. For example, materialproperty enhancement to a compositionally identical base epoxy portionmay be achieved by post-treating the base epoxy portion after dispensinginto the mold. Such post-treatment may be differential heating (such asby having established a temperature gradient in the mold or by using astratified oven or stratified heated airflow). Such pretreatment mayadditionally or alternatively be differential irradiation with zonal IR,UV, visible, microwave, X-ray, or other electromagnetic radiation sourceor by E-beam or other particle beam. Also, certain micro-structuraleffects (grain migration, lamination, orientation, size, agglomeration,etc.) may be effected by exposing all or part of the device materials toelectric fields, magnetic fields, centrifugal/centripetal forces orgravity before, during, or after dispensing.

FIG. 12 shows a radiation emitter device constructed in accordance witha third embodiment of the present invention. The device 200 shown inFIG. 12 is configured to have the same size and shape as HewlettPackard's SuperFlux or Piranha devices. As shown, device 200 differs,however, in that it incorporates an encapsulant 212 having a first zone230 and a second zone 232 applied in a similar manner as applied in thefirst two embodiments.

FIGS. 13 and 14 show a radiation emitter device 250 constructed inaccordance with a fourth embodiment of the present invention. Asapparent from FIGS. 13 and 14, this fourth embodiment is intended toresemble, in both size and shape, the configuration of Hewlett Packard'sSnapLED device. By configuring various embodiments of the invention toresemble conventional products in size and shape, the devices of thepresent invention may be readily substituted for the conventionaldevices without requiring any modification to equipment used to populatea circuit board. Additionally, by configuring these embodiments toresemble conventional structures, the same encapsulant mold cavitiesthat are used to make the conventional devices may be used to make theinventive embodiments thereby eliminating the need to significantlymodify the apparatus used to manufacture these radiation emitterdevices.

FIG. 15 shows a radiation emitter device 300 constructed in accordancewith a fifth embodiment of the present invention. Device 300 includes anadditional heat extraction member 310 that extends outward from theencapsulant separate and apart from the electrical leads. Other suitableconstructions utilizing a heat exaction member are disclosed in commonlyassigned above-referenced U.S. Pat. No. 6,335,548, the entire disclosureof which is incorporated herein by reference.

With the construction shown in FIG. 15, it may be desirable to form amicro-groove lens, such as a Fresnel lens directly in the encapsulant.This is particularly advantageous when LED chips of different colors areprovided in a single discrete LED device where the light from the chipsis to mix to form another color such as white light. A reflectiveelement may also be secured to the light output surface of the device tofurther modify the light emitted from the device. An example of an LEDdevice having a micro-groove lens and such a reflective element isdisclosed in commonly assigned U.S. Patent Application No. 60/270,054(unofficial) entitled RADIATION EMITTER DEVICE HAVING A MICRO-GROOVELENS, filed on Feb. 19, 2001 by John K. Roberts, the entire disclosureof which is incorporated herein by reference.

The invention will be further clarified by the following example, whichis intended to be exemplary of the invention and is not intended in anymanner to limit the invention.

EXAMPLE

To demonstrate the effectiveness of the present invention, two LEDdevices were constructed and tested. The first LED device was aconventional T-1¾ LED device, while the second LED device had anidentical construction with the exception that encapsulant 12 included asecond zone 32 as disclosed above with respect to the first embodimentof the present invention. The conventional T-1¾ LED device wasconstructed using HYSOL® OS4000 transparent epoxy available from DexterElectronic Materials Division. The inventive T-1¾ LED device wasconstructed using the same transparent epoxy for first zone 30. Thesecond zone 32, however, was formed using HYSOL® EO0123 castingcompound, which is also available from Dexter. The two LED devices werethen operated by DC operation at room temperature, and their averagenormalized luminous flux was measured and plotted in the graph shown inFIG. 16. As apparent from this graph, the inventive LED device had muchgreater luminous flux, particularly at higher powers.

It should be understood that, for this sample, increased illuminance ateach indicated power level for the inventive LED device relative to theconventional LED device is an indication of reduced junction operatingtemperature and reduced assembly thermal resistance.

While the present invention is generally described as employing two orthree zones in the encapsulants that are arranged substantiallyvertically when the primary optical axis of the device is vertical, itwill be appreciated that the zones may be oriented with respect to eachother to the left or right of the central optical axis or one inside oroutside the other. Such an inside/outside arrangement of the encapsulantzones may be affected by achieving a “cure gradient” from the outside tothe inside whereby the inside is not fully cured and is left soft for awhile into the life of the radiation emitter device. Such aconfiguration may also be used by curing the inside of the LED using theheat generated by the radiation emitter 35 itself. This may beadvantageous when low residual mechanical stress is desired.

In certain embodiments of the present invention, thermal resistance fromthe radiation emitter junction to the ambient environment is reducedwithout reducing thermal resistance (junction to lead), and therefore,better operating temperature (i.e., lower operating temperature) at agiven power may be achieved without increased susceptibility tolead-soldering thermal damage.

The above description is considered that of the preferred embodimentsonly. Modifications of the invention will occur to those skilled in theart and to those who make or use the invention. Therefore, it isunderstood that the embodiments shown in the drawings and describedabove are merely for illustrative purposes and not intended to limit thescope of the invention, which is defined by the following claims asinterpreted according to the principles of patent law, including thedoctrine of equivalents.

The invention claimed is:
 1. A radiation emitting device comprising: at least one radiation emitter; first and second electrical leads electrically coupled to said radiation emitter; and an integral encapsulant configured to encapsulate said radiation emitter and a portion of said first and second electrical leads, said encapsulant having at least a first zone and a second zone, the second zone exhibiting at least one different characteristic from the first zone, wherein said at least one different characteristic is that said second zone has a higher thermal conductivity than said first zone, and wherein, in both said zones, said encapsulant is made of a material with a relatively high electrical resistivity.
 2. The radiation emitting device of claim 1, wherein said at least one different characteristic is a physical characteristic.
 3. The radiation emitting device of claim 1, wherein said at least one different characteristic is an optical characteristic.
 4. The radiation emitting device of claim 3, wherein said at least one different optical characteristic is transparency.
 5. The radiation emitting device of claim 3, wherein said at least one different optical characteristic is diffusivity.
 6. The radiation emitting device of claim 1, wherein said at least one different characteristic is a thermal characteristic.
 7. The radiation emitting device of claim 6, wherein said at least one different thermal characteristic is thermal expansion coefficient.
 8. The radiation emitting device of claim 6, wherein said at least one different thermal characteristic is specific heat.
 9. The radiation emitting device of claim 6, wherein said at least one different thermal characteristic is glass transition temperature.
 10. The radiation emitting device of claim 1, wherein said at least one different characteristic is a structural characteristic.
 11. The radiation emitting device of claim 10, wherein said at least one different characteristic includes at least one of tensile strength and compression strength.
 12. The radiation emitting device of claim 1, wherein said at least one different characteristic is a compositional characteristic.
 13. The radiation emitting device of claim 1, wherein said radiation emitter is mounted on one of said first and second electrical leads.
 14. The radiation emitting device of claim 1 and further including a wire bond extending from one of said first and second electrical leads to said radiation emitter.
 15. The radiation emitting device of claim 1, wherein the at cast one different characteristic includes at least one of the following: mechanical strength, thermal capacity, specific heat, coefficient of thermal expansion, adhesion, oxygen impermeability, moisture impermeability, and transmittance for radiation emitted from said radiation emitter.
 16. The radiation emitting device of claim 1, wherein one of said zones is at least partially transparent to radiation emitted from said radiation emitter.
 17. The radiation emitting device of claim 1, wherein a region of said encapsulant is configured to function as a lens.
 18. The radiation emitting device of claim 1, wherein a region of said first zone of said encapsulant is configured to function as a lens.
 19. The radiation emitting device of claim 18, wherein said second zone of said encapsulant is configured to retain said electrical leads.
 20. The radiation emitting device of claim 19, wherein said lens is formed on a side of said encapsulant different from any sides from which said electrical leads extend.
 21. The radiation emitting device of claim 20, wherein the side on which said lens is formed is opposite a side from which said electrical leads extend.
 22. The radiation emitting device of claim 20, wherein the side on which said lens is formed is adjacent sides from which said electrical leads extend.
 23. The radiation emitting device of claim 1, wherein said electrical leads are wave-solderable.
 24. The radiation emitting device of claim 1, wherein said radiation emitter is an LED chip.
 25. The radiation emitting device of claim 1, wherein said radiation emitter is a PLED.
 26. The radiation emitting device of claim 1, wherein said radiation emitter is an OLED.
 27. The radiation emitting device of claim 1, wherein said radiation emitter is an LEP.
 28. The radiation emitting device of claim 1, wherein said radiation emitter is a semiconductor optical radiation emitter.
 29. The radiation emiting device of claim 1, wherein said at least one radiation emitter includes a first radiation emitter and said device further comprises a second radiation emitter, both of said first and second radiation emitters are encapsulated by said encapsulant.
 30. The radiation emitting device of claim 29, wherein said second radiation emitter is an LED chip.
 31. The radiation emitting device of claim 29, wherein said second radiation emitter is a photoluminescent emitter.
 32. The radiation emitting device of claim 29, wherein the first radiation emitter is an LED chip and said second radiation emitter is a photoluminescent emitter.
 33. The radiation emitting device of claim 1, wherein said first zone of said encapsulant is optically transparent and extends from said radiation emitter to a light output surface of said encapsulant.
 34. The radiation emitting device of claim 1, wherein said second zone has a higher thermal capacity than said first zone.
 35. The radiation emitting device of claim 1, wherein said second zone has a greater mechanical strength than said first zone.
 36. The radiation emitting device of claim 1, wherein said second zone has a lower coefficient of thermal expansion than said first zone.
 37. The radiation emitting device of claim 1, wherein said second zone has a greater adhesion strength than said first zone with respect to said electrical leads.
 38. The radiation emitting device of claim 1, wherein said second zone has lower oxygen permeability than said first zone.
 39. The radiation emitting device of claim 1, wherein said second zone has lower moisture permeability than said first zone.
 40. The radiation emitting device of claim 1, wherein said second zone has a higher specific heat than said first zone.
 41. The radiation emitting device of claim 1, wherein said first and second zones of said encapsulant are cohesively bonded.
 42. The radiation emitting device of claim 1, wherein said integral encapsulant is heterogeneous.
 43. The radiation emitting device of claim 1, wherein said electrical leads extend from said encapsulant substantially non-perpendicular to the optical axis of the device.
 44. The radiation emitting device of claim 1, wherein said first and second zones are made of different compositions and wherein a gradient mix of the different compositions exists between said zones.
 45. The radiation emitting device of claim 1, wherein both said zones of said encapsulant are made of a thermoset material.
 46. The radiation emitting device of claim 1, wherein said encapsulant is integrally molded.
 47. The radiation emitting device of claim 1 and further including a glob-top disposed over said radiation emitter.
 48. The radiation emitting device of claim 47, wherein said glob-top includes a photoluminescent material.
 49. The radiation emitting device of claim 1, wherein, in said second zone, said encapsulant physically contacts said electrical leads.
 50. The radiation emitting device of claim 1, wherein, in said second zone, said encapsulant is an epoxy.
 51. An optical radiation emitting device comprising: an optical radiation emitter; first and second electrical leads electrically coupled to said optical radiation emitter; and an integrally molded encapsulant configured to encapsulate said optical radiation emitter and a portion of said first and second electrical leads, said encapsulant being made of a first composition that is substantially optically transparent to radiation emitted by said radiation emitter, and a second composition having different properties than said first composition including a higher thermal conductivity than said first composition, wherein said encapsulant is made of a material with a relatively high electrical resistivity.
 52. The radiation emitting device of claim 51, wherein said first and second compositions are substantially segregated into different regions of said encapsulant.
 53. The radiation emitting device of claim 51, wherein said second composition is substantially opaque.
 54. The radiation emitting device of claim 51, wherein said first composition is substantially transparent.
 55. The radiation emitting device of claim 51, wherein said second composition has a higher thermal capacity than said first composition.
 56. The radiation emitting device of claim 51, wherein said second composition has a greater mechanical strength than said first composition.
 57. The radiation emitting device of claim 51, wherein said second composition has a lower coefficient of thermal expansion than said first composition.
 58. The radiation emitting device of claim 51, wherein said second composition has a greater adhesion strength than said first composition with respect to said electrical leads.
 59. The radiation emitting device of claim 51, wherein said second composition has lower oxygen permeability than said first composition.
 60. The radiation emitting device of claim 51, wherein said second composition has lower moisture permeability than said first composition.
 61. The radiation emitting device of claim 51, wherein said second composition has a higher specific heat than said first composition.
 62. The radiation emitting device of claim 51, wherein said first and second compositions are substantially segregated into different zones of said encapsulant and are cohesively bonded at an interface of those zones.
 63. The radiation emitting device of claim 51, wherein said optical radiation emitter is mounted on one of said first and second electrical leads.
 64. The radiation emitting device of claim 51, wherein said radiation emitter is an LED chip.
 65. The radiation emitting device of claim 51, wherein said radiation emitter is a semiconductor optical radiation emitter.
 66. The radiation emitting device of claim 51 and further including a glob-top disposed over said optical radiation emitter.
 67. The radiation emitting device of claim 66, wherein said glob-top includes a photoluminescent material.
 68. The radiation emitting device of claim 51, wherein said second composition physically contacts said electrical leads.
 69. The radiation emitting device of claim 51, wherein said second composition is an epoxy.
 70. A light emitting device comprising: at lean one LED chip; first and second electrical leads electrically coupled to said LED chip; and an integral encapsulant configured to encapsulate said LED chip and a portion of said first and second electrical leads, said encapsulant having at least a transparent first zone and a second zone, the second zone having a higher thermal conductivity than said first zone, wherein, in both said zones, said encapsulant is made of a material with a relatively high electrical resistivity.
 71. The light emitting device of claim 70, wherein, in said second zone, said encapsulant physically contacts said electrical leads.
 72. The light emitting device of claim 70, wherein, in said second zone, said encapsulant is an epoxy. 